In injection devices where the drive sleeve is coupled to the number sleeve via a clutch during dialing (dose setting) and coupled to the housing during dispense, via a clicker, and where coupling and decoupling is accomplished by a movement of the clutch member between a dialing (dose setting) and dispense position, it is important to ensure that in both general and misuse scenarios at extremes of tolerance, the user cannot decouple the drive sleeve from the number sleeve without first ensuring that it is sufficiently coupled to the housing.
If at any point the drive sleeve is neither coupled to the number sleeve nor to the housing with adequate strength, it may be possible for the user to rotate the drive sleeve relative to the number sleeve and housing. In devices where the drive sleeve has a threaded connection or splined connection to a piston rod that moves axially to expel the drug contents in the cartridge, rotating the drive sleeve relative to both the number sleeve and housing allows the user to back off the piston rod from the cartridge bung without decrementing the dialed dose, and this may lead to a severe under dose on the subsequent dose. If the drive sleeve is rotated in the opposite sense, it may also allow the user to expel drug without decrementing the dialed dose, leading to loss of drug and possible confusion.
FIG. 1 shows a known dose setting mechanism 1 where a spring 7 biases the clicker 6 into engagement with the clutch 5 and the clutch 5 into engagement with the number sleeve 3. Clicker teeth 6a, 6b of axial height ‘Y1’ engage between the clicker 6 and clutch 5 components and the clutch teeth 5a, 5b of axial height ‘Z1’ engage between the clutch 5 and the number sleeve 3 components. In this ‘At rest’ state a gap of ‘X1’ exists which is defined as the amount that the clicker 5 can move axially towards the drive sleeve 4 before the spring 7 is compressed to a solid state and prevents further axial movement.
This gap ‘X1’ has a total tolerance of ‘T’ which is made up from the addition of the individual tolerances of the component parts, clicker axial length, clutch axial length, number sleeve flange thickness, drive sleeve axial length and the spring height when compressed solid.
For the device to be dialable, the gap ‘X1’ in its minimum tolerance condition, X1-T, must still be large enough to allow the clicker teeth 6a, 6b to ride over each other during dialing so that the clutch 5 and hence drive sleeve 4 can rotate relative to the number sleeve 3. Hence the device must comply with the equationX1−T>Y1.
Similarly in order for the device to be able to dispense the gap X must be large enough to allow the clutch teeth 5a, 5b between the clutch 5 and number sleeve 3 to disengage. In this case, the device must comply with the equationX1−T>Z1.
In addition, to ensure that the drive sleeve 4 is either rotationally coupled via the clutch 5 to the number sleeve 3 or rotationally coupled to the housing 2 via the clutch 5 and clicker 6, and is not in some indeterminate state where it can rotate relative to both parts, the device must comply with the equationX1+T<Z1+Y1−K 
where K is defined as the minimum overlap between either the clicker teeth 6a, 6b or the clutch teeth 5a, 5b to ensure that the drive sleeve 4 has sufficient engagement with one of these sets of teeth so as not to rotate relative to both sets. This minimum overlap K will have to be larger if the device is to be able to withstand rotation of the drive sleeve 4 relative to the housing 2 and the number sleeve 3 under a reasonable user applied torque. Adding the tolerance T to X1 allows to define a worst case combination of parts for this failure mode.
Combining the above three equations,X1−T>Z1  (1)X1−T>Y1  (2)andX1+T<Z1+Y1−K  (3)
one can substitute from (1) X1=Z1+T into (3) to giveY1>2*T+K 
and from (2) one can substitute X1=Y1+T into (3) to giveZ1>2*T+K. 
Due to the long tolerance chain identified above this total stack tolerance of T may be as much as T=0.4 mm, and to ensure adequate strength K=0.4 mm as well. In this case Y1 must be greater than 2*0.4 mm+0.4 mm=1.2 mm. The clicker teeth 6a, 6b however have e.g. 24 positions per turn and hence 24 teeth. A height of 1.2 mm would require the clicker teeth to have a very steep flank angle given the restricted diameter of the part, and this flank angle would either make dialing not possible or give a very high dialing torque. In prototypes tested the clicker teeth height was only 0.7 mm and this gave a reasonable dialing torque.
WO 2006/037434 A1 discloses a drive mechanism for a drug delivery device. The embodiment of FIGS. 2 to 6 of this document includes a first clutch between a dose setting dial and an inner cylinder and a second clutch between the inner cylinder and a release knob. However, WO 2006/037434 A1 does not teach that at any time during operation either the first clutch couples the dose setting member a the drive member and/or the second clutch couples a drive member to a clicker component. Moreover, a clicker producing a tactile and/or audible feedback is not described in this document. In addition, in the mechanism of WO 2006/037434 A1 there is no clutch member located between the dose setting member to a drive member which clutch member is movable relative to these components in an axial direction.